The present invention relates in general to a thermal dot printer, and in particular to a thermal dot printer and a method for controlling a thermal dot printer using interleave pulse modulation for heating up the printing elements of the thermal printhead of the thermal dot printer.
Various kinds of dot printers are known in the art. So-called thermal dot printers employ thermal energy to form images or characters on a media. Generally speaking, such thermal dot printers operate by either applying thermal energy to the media or to a heat sensitive coating on the surface of the media to alter the characteristics of the media or the characteristics of the heat sensitive coating, or by thermally energizing a thermally sensitive hot melt wax ink ribbon to transfer ink from the ribbon to the media.
Typically, a thermal dot printer contains a thermal printhead for printing characters or images on a media, a drive system for moving the paper across the printhead, a print logic for outputting character print signals to the printhead, and optionally, a drive system for moving the printhead across the paper.
The thermal printhead usually includes a plurality of print positions arranged in either vertical or horizontal lines. Each print position includes a printing element connected to wires. When electrical power is applied to the wires, the printing element increases in temperature. At a certain temperature, the printing element causes a visible dot to appear on the media being printed. A group of closely spaced dots represents a character or symbol.
When using a conventional thermal matrix printer in which the printing elements of the thermal matrix printhead are arranged in the form of a matrix, the printing elements are selectively heated to form a character as the thermal matrix printhead travels in the printing direction at a predetermined pitch across the paper or medium path. As such, one character is formed by the group of dots each time the thermal matrix printhead is traveled by a predetermined number of dots. Once an entire row of characters is printed, the paper advances so that another row of information can be printed as the process is repeated.
While conventional thermal matrix printers are still used in some applications, modern printers are more likely to employ a thermal in-line printhead. A thermal in-line printhead is a stationary printhead that uses a series of dot printing elements configured in a horizontal line across the width of the paper. As such, the printhead remains stationary with respect to the paper. The number of printing elements is a function of the print quality and the width of the paper. As opposed to a thermal matrix printer, which prints a single character then moves a predetermined amount before printing another character, an in-line printhead selectively prints a horizontal row of dots across the paper at once. The drive system, comprising a stepper motor and a system of gears and rollers, continuously moves the print media at a predetermined rate along a paper path allowing the sequential printing of multiple rows of dots. Thus, all the characters in the row are formed as multiple rows of dots are printed across the media.
A typical in-line printhead may have several hundred print elements. Typically these elements are divided into groups of elements. For example, a printhead configuration of 640 elements might be divided into five groups of 128 elements. Each of the five groups could be activated separately with separate control lines. Separate control lines are desirable because the power required to activate all 640 elements exceeds the capacity of power supplies typically used in in-line printers. With typical power supplies, three groups of elements can be activated at a time. Thus, in a printhead configuration of 640 elements, it is possible to combine the control lines into two xe2x80x9cbanksxe2x80x9d of groups. The first bank contains three groups of elements (or 384 elements comprising the left column of the paper) and the second bank contains two groups of elements (or 296 elements comprising the right column of the paper). Both banks of elements are used to print one row of 640 dots.
For printing a first row of dots, therefore, a first set of printing elements is activated in the first bank of elements. After printing the first set of dots, the first set of printing elements is deactivated, and a second set of printing elements from the second bank of elements has to be activated for printing the rest of the row of dots. After printing the second set of dots, the second set of printing elements is deactivated. This process is repeated to print another row of dots until the entire row of characters is printed on the media. During the printing process, the print media is continually moving across a media or paper path. The rate that the dots are printed out corresponds to the distance that it is moved while printing. Once the row of characters is printed, the paper is advanced so that another row of characters or other information can be printed as the process is repeated.
The size and shape of the dots is a function of the shape of the printing element, the temperature of the printing element and the length of time the printing element is applied to the media or to the ribbon.
For printing a dot on the media, the respective printing element has to be supplied with electrical power for a sufficient length of time to heat up the printing element up to a predetermined printing temperature and to keep this printing temperature for a predetermined length of time. More precisely, for printing a dot, the printing element must rise to the predetermined printing temperature which is sufficient to alter the characteristics of the medium and stay at this temperature for a predetermined period of time to complete the printing of the dot. Consequently, the temperature of the printing element and the amount of heat applied by this printing element is dependent upon the level of drive current supplied to the printing element and the length of time the drive current is being supplied to the printing element.
A well known method for heating up the printing elements is xe2x80x9cnormal pulsingxe2x80x9d, i.e. a printing element is supplied with a single long drive current pulse of predetermined pulse width and pulse amplitude. The graph of such a pulse is illustrated in FIG. 3. The y or vertical axis represents the pulse amplitude and the x or horizontal axis represents units of time. In a thermal in-line printer, normal pulsing is performed by supplying a long drive current pulse to a first set of printing elements in the first bank of dots, and then supplying a long drive current pulse to a second set of printing elements in a second bank of dots. Consequently, not every element will be activated in a bank of elements. A xe2x80x9csetxe2x80x9d of elements refers to those elements within a particular bank of elements that will be activated or used in the process to print a row of dots. The set of elements will vary within the banks depending on the characters in the row to be printed.
FIG. 3 shows an example for xe2x80x9cnormal pulsingxe2x80x9d of the current pulses to two successive sets of printing elements in a printhead. The pulse curve for the first set in the first bank of elements is 304. The pulse curve for the second set in the second bank of elements is 306. As an example of normal pulsing, a printing element of the first set of printing elements is turned on for 10 units of time (first current pulse 301). After completion of pulse 301, i.e. when the first set of dots is printed, a second current pulse 302 is supplied to a second set of printing elements. Since the row of dots is comprised of both sets of dots, the entire row is printed once both sets of dots have been printed.
The process is repeated for the next row of dots until the entire row of characters is printed on the media. During this time, the paper or media is constantly moving along the paper path. The sets of dots are printed separately while the media is moving. Thus, there is a slight shift between the sets of dots. However, the distance between the dot sets is small and is not perceptible to the human eye.
Heating the printing elements by using normal pulsing (i.e., by supplying the printing elements with a single long drive current pulse) has the drawback of making some printing elements hotter than they need to be. A first portion of pulse 301 is used to heat up the respective printing element to a predetermined printing temperature necessary for altering the characteristics of the media or for melting the ink of the ribbon. This portion is represented in FIG. 3 as that portion of pulse 301 between points 305 and 307 once the correct temperature has been achieved. A second portion of pulse 301 is necessary to apply the heat of the printing elements to the media for a length of time long enough for printing a dot on the media. In FIG. 3, this portion is represented by the portion from point 307 to point 309. During the second portion of the pulse 301, the temperatures of the printing elements are increasing. By the end of pulse 301 (i.e., at point 309) the printing elements have overheated or become xe2x80x9chotterxe2x80x9d than they need to be. The overheating of the printing elements stresses the printhead which may result in shortening the working life of the thermal printhead. It also creates xe2x80x9cbleedingxe2x80x9d of the dots into the next dot row because the print elements exceed a temperature that is above the minimum print temperature necessary for printing. This temperature is maintained even after the electrical signal is removed from the element.
The standard solution for this problem is pulse modulation. In a first step, a first set of printing elements is supplied with a first current pulse to heat up the printing element to the printing temperature. Then, in a second step, the first set of printing elements is supplied with a sequence of shorter current pulses to maintain the temperature of the first set of printing elements at the printing temperature. The graph of this procedure for printing two banks of elements is illustrated in FIG. 4. After completion of the second step, i.e. after printing a first set of dots from the first bank of dots, a long pulse and series of short bursts are then applied to the second set of elements in the second bank in order to complete printing of the row of dots.
FIG. 4 shows an example of xe2x80x9cpulse modulationxe2x80x9d for printing two banks of elements. In pulse modulation, printing elements in the first set of printing elements are turned on for 6 units of time (first current pulse 401). By the end of pulse 401 (at point 405) these printing elements have reached their printing temperature. Thereafter, these printing elements are supplied with a sequence of short current pulses 402a to 402d to maintain the printing temperature of these printing elements to just above the minimum temperature required for printing. Overheating of these printing elements are, therefore, avoided. After completion of pulse 402d, the first set of dots is printed. Then, a second current pulse 403 is supplied to a second set of printing elements in the second bank to complete the printing of a single row of dots. By the end of pulse 403 these printing elements have reached their printing temperature. Thereafter, these printing elements are supplied with a sequence of short current pulses 404a to 404d to maintain the printing temperature of these printing elements. Thus, sets of dots from both banks have been printed which completes the row of dots to be printed. Simultaneously, the paper is continuously moving along the paper path. This process is repeated for the next row of dots until the entire row of characters is printed on the media. Because pulse modulation keeps the elements to more of a constant temperature, the stress on the printhead is reduced. Furthermore, bleeding of the dots is also reduced which increases print quality.
As can be seen when comparing FIG. 3 to FIG. 4, the overall xe2x80x9cON-timexe2x80x9d, i.e the sum of the pulse width (ON-time) of pulse 401 and the pulse widths (ON-times) of the sequence of pulses 402a through 402d, is approximately the same as the pulse width (ON-time) of the long pulse 301 used in xe2x80x9cnormal pulsingxe2x80x9d. However, because the pulse modulation has some xe2x80x9cOFF-timesxe2x80x9d during the second step, the overall time required for xe2x80x9cpulse modulationxe2x80x9d is greater than the overall time for xe2x80x9cnormal pulsingxe2x80x9d. This reduces the maximum speed of printing. In the example of FIG. 4, pulse modulation takes 40% more time than the normal pulsing illustrated in FIG. 3 obviously reducing the maximum speed of the printer.
What is needed, therefore, is a device and method which economically and simply increases the speed of printing by shortening the time period for activating the printing elements, and which prevents the stressing of the print elements due to excessive element temperature, and which prevents the creation of xe2x80x9cbleedxe2x80x9d of the dots into the next dot row due to excessive temperature of the print element.
The previously mentioned needs are fulfilled with the present invention. Accordingly, there is provided a thermal printer composed of an in-line thermal printhead for printing symbols or characters on a media surface as a series of dots, a drive system for moving the paper across the printhead, and print logic for outputting print signals to the printhead such that the printing elements are heated by way of interleave pulse modulation.
By mirror imaging the second pulse train and interleaving the short pulses for each bank of printing elements, the overall time for heating and retaining the printing temperature of successive sets of printing elements is reduced.
In an embodiment of the invention, in a first step, a first set of printing elements is supplied with a first current signal for heating the first set of printing elements up to the printing temperature. Then, in a second step, the first set of printing elements is supplied with a second current signal for retaining the printing temperature of the first set of printing elements for printing a first bank of dots. In a third step, before completion of supplying the second current signal to the first set of printing elements, a third current signal is supplied to a second set of printing elements for heating the second set of printing elements up to the printing temperature. After completion of the third current signal, a fourth current signal is supplied to the second set of printing elements to retain the printing temperature of the second set of printing elements for printing a second bank of dots. The row of dots is printed when both banks of dots have been printed and the paper is advanced for printing a next rows of dots. This process is repeated until the entire row of characters is printed on the media.
In an embodiment of the invention, the third current signal is supplied to the second set of printing elements after completion of the first current signal which is supplied to the first set of printing elements. The fourth current signal is supplied to the second set of printing elements after completion of the second current signal which is supplied to the first set of printing elements.
In an embodiment of the invention, at least one of the above current signals is a long current pulse, and the remaining current signals consist of a sequence of short current pulses. The short current pulses simultaneously supplied to the first and second set of printing elements are synchronized in a way that the ON-times of the short current pulses of the one pulse sequence are concurrent with the OFF-times of short current pulses of the other pulse sequence.
The interleave pulse modulation gives the printer the advantages of pulse modulation (longer printhead life and less xe2x80x9cbleedxe2x80x9d), without the disadvantage of reducing the speed of printing.
These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to note that the drawings are not intended to represent the only form of the invention.